1. Field of the Invention
The present invention relates generally to power controllers, but more particularly to an overcurrent protection system that uses a drive circuit to power an overcurrent sensing/inverse time delay circuit, to drive a power switch, and to couple an overcurrent fault signal across electrical isolation interface(s) thereof to a logic circuit.
2. Description of the Prior Art
Electrical power systems, such as single or multi-phase ac or dc voltage types, are required to be operative over a range of conditions limited by the current handling capability of the wiring and loads. Due to cost and size consideration, it is frequently desirable to minimize the power handling capability of the system. However, for reasons of safety, it is required to provide devices for the system to determine when current levels approach an overload condition, i.e., the point at which wiring and/or components may be damaged. As an example, in electrical power systems for aircraft electrical systems, the need for minimal size and weight, accompanied by high reliability and safety, is particularly acute.
In the past, overcurrent protection systems have been used to protect the loads, wiring and/or components of electrical power systems from the consequences of faults and overloads. One such circuit is disclosed in U.S. Pat. No. 3,925,709 to Mitchell et al., filed Aug. 13, 1973, entitled, "Overcurrent Protection Circuit for AC Systems" and assigned to Westinghouse Electric Corp. Disclosed in Mitchell et al. is an overcurrent protection circuit for use with ac systems. The circuit developes a full wave dc sensing voltage whose peak magnitude is proportional to the load current of the ac system to be protected. The foregoing dc sensing voltage is then applied to an optical isolator having a secondary circuit which includes an optically responsive device having a conduction threshold voltage in a predetermined relationship to the dc sensing voltage magnitude for normal load current. The secondary circuit further includes a time delay network and a voltage level detector which, in turn are responsive to the current through the optically responsive device resulting in an overcurrent trip signal, at the output of the voltage level detector, upon the occurrence of predetermined overcurrent conditions in the load and wiring.
Previous overcurrent protection circuits, like that disclosed in Mitchell et al., and other prior art, and used in power controllers have limitations in one or more of the following areas: coupling of the overcurrent fault or trip signal from the power line to a lower potential for the logic circuits; increased power dissipation incurred by the element used to sense the load current; and/or being applicable to either ac or dc power controllers.
Consequently, there is a need in the prior art to overcome the foregoing limitations in overcurrent protection circuits and to include the improvements in a single circuit implementation.
The prior art, as indicated hereinabove, include advances in overcurrent protection circuits and systems; however, insofar as can be determined, no prior art overcurrent protection circuit or system incorporates all of the features and advantages of the present invention.